Although noise creates a problem in any video system, the normal picture quality of commercial television sets is so low that the viewer generally tolerates a fair amount of noise and signal distortion. In the large-screen television art, the videotape-to-film or electronic film-to-film conversion art, and the astronautical communications art, the noise problem assumes major proportions, particularly where color signals are involved. Noise in this discussion encompasses all kinds of random signal degradation. In film-to-film conversion, it may be caused by the grain structure of the original film; in any system, it may be caused by supurious electronic signals; and in color video systems, it may be caused by incomplete separation of the luminance and chroma signals.
It has previously been proposed to reduce noise in video systems by comparing information from adjacent portions of a video image, an e.g., by comparing successive lines of the same image. Due to the variation of image information from line to line, the applicability of this technique is severely limited, quite aside from the fact that it degrades the sharpness of the picture.
In color television, the standard video signal contains a luminance information component in the form of wide-band amplitude modulation of the video carrier, and a chroma information component in the form of modulation of a color subcarrier whose phase alternates between 0.degree. and 180.degree. between successive lines of a field and between corresponding lines of corresponding fields of successive frames of the picture. For color decoding purposes, these two components must be separated and individually processed by appropriate circuits.
The simplest way of separating the luminance and chroma components is to filter out the color subcarrier and its sidebands. This approach, however, removes valuable high-frequency information from the luminance component and produces spurious chroma information due to high-frequency luminance modulation. In the prior art, these disadvantages have been eliminated by using a so-called comb filter which compares two successive lines of the image (whose chroma components, as stated above, are of opposite phase) and produces a pure luminance component by addition and a pure chroma component by subtraction. The disadvantage of the comb filter is that wherever there is a substantial color or intensity change in the image from one line to the next, edge sharpness is lost and color or intensity distortion occurs due to incomplete separation.